In many hydrometallurgical processes, the established reduction step for producing metallic nickel powder involves a nickel sulphate feed solution, typically an aqueous feed solution, to which ammonium sulphate (Amsul) is added and which is adjusted with ammonia to produce a nickel diammine sulphate solution. In general, in industrial practice, the ammonia adjustment is sufficient to allow for the quantitative formation of the nickel diammine complex in solution, and the ammonium sulphate addition is sufficient to stabilize the nickel diammine complex to minimize the precipitation of nickel as the hydroxide. The thus stabilized nickel diammine sulphate solution, i.e., adjusted in composition so as to minimize the risk of precipitate formation, is then contacted with hydrogen at elevated temperature and pressure in an autoclave to reduce the nickel from solution in the form of an elemental nickel powder. The reduction process typically includes two steps. The first step is a nucleation step in which an initial reduction of nickel produces finely-divided material termed a seed material. This seed material is used in the first of a following series of densification cycles, wherein the nickel powder in the vessel is allowed to settle, the essentially nickel-free reduction end solution (RES) is discharged from the vessel, fresh nickel diammine sulphate solution is introduced to the vessel, and reduction with hydrogen is repeated through multiple densification cycles. In each densification cycle, additional nickel is reduced onto the previously formed metallic nickel particles, causing such particles to grow in size, until the target size distribution of the nickel powder product is obtained.
Exemplary patents having teachings directed to reducing nickel from solution to produce nickel powder products, or to nickel reduction with nucleation and densification cycles include U.S. Pat. Nos. 2,734,821; 2,767,083; 2,853,374; and 3,816,098.
The above-mentioned nickel sulphate solution may be prepared in a number of ways, or result from a number of processes, before its adjustment in terms of ammonia and ammonium sulphate additions. For example, in the recovery of nickel metal from laterite ore, a High Pressure Acid Leach (HPAL) process may be used to prepare a nickel sulphate solution, as practised on an industrial scale by Ambatovy in Madagascar, amongst others. In this process, nickel and cobalt in laterite ore, containing about 1 wt % Ni, 0.1 wt % Co and 1 to 3 wt % silicon, are extracted into solution in a high pressure acid leach step. Following partial neutralization of the solution with limestone, a nickel-cobalt mixed sulphide intermediate material may be produced by precipitation of the nickel and cobalt from the partially neutralized solution as their sulphides, by addition of hydrogen sulphide gas. The mixed sulphide intermediate is then leached in the presence of oxygen, the resulting nickel-cobalt solution is purified by iron and copper removal, and solvent extraction then separates cobalt from nickel. The purified nickel sulphate solution from solvent extraction is then adjusted, as described above, by ammonia and ammonium sulphate (Amsul) addition to form a stabilized nickel diammine solution. Nickel powder is then produced by hydrogen reduction of the diammine solution, as set out above.
During this HPAL process, a small fraction of the silicon that is initially leached from the laterite ore in the high pressure acid leach step reports to the mixed sulphide intermediate. A portion of the silicon in the mixed sulphide intermediate is extracted in the oxidizing leach and only a small fraction of this is removed in the iron and copper removal steps. Hence, a portion of the silicon may be carried over into the diammine solution after purification and solvent extraction and is precipitated as an undesired impurity with the nickel powder, lowering the commercial value of the product. The quantity of silicon precipitated to the nickel powder is roughly proportional to the silicon concentration in the solution generated by the oxidizing leach of the sulphide intermediate. Typically, plants using this process do not have a controlled method of removing silicon from this solution. As a result, if the silicon concentration present in the mixed sulphide intermediate is high, the silicon concentration in the corresponding nickel powder will be high. As an example, nickel powder produced by this process during the early months of operation of the Ambatovy refinery frequently contained in excess of 0.02 wt % silicon, which, although significantly lower than the relative content of silicon to nickel in the original ore, is significantly higher than the production specification for nickel powder that was targeted (<0.005 wt % Si).
In order to achieve low silicon levels in the nickel powder product, care must be taken to minimize the transfer of silicon to the HPAL leach solution, from the HPAL leach solution to the sulphide intermediate, from the sulphide intermediate to the oxidative leach solution, and so on. U.S. Pat. No. 7,387,767 discloses an HPAL process for laterite ore, including efforts to decrease silicon and other impurities in the mixed sulphide intermediate.
Control of the silicon content of the final nickel powder product requires control of silicon precipitation behaviour in both the nucleation and the densification steps, neither of which is well understood in the prior art processes.
U.S. Pat. No. 4,149,875 discloses a process to purify nickel or cobalt metal powders containing high amounts of silicon impurities using a sodium hydroxide wash at elevated temperatures. While post-washing of the metal powders in this manner has not generally been found to be a satisfactory solution to the problem, this patent discusses the difficulties faced in the hydrometallurgical industry in controlling the level of silicon impurities in the nickel and/or cobalt powder end product.